Engine oil, sometimes also referred to as engine lubrication oil or engine lube oil, is continuously fed from an oil rail, or oil gallery to engine parts needing lubrication. For example, it is typically necessary to lubricate parts such as crank shaft and connecting rod bearings, turbochargers and, in some engines, piston cooling jets. An engine oil system is additionally often provided with an engine oil cooler to control the temperature of the engine oil. As engine oil temperature increases, the viscosity of the engine oil decreases, that is the viscosity number of the engine oil decreases, and the engine oil becomes thinner. When the viscosity of the oil is lower, friction between lubricated parts is reduced, but engine durability may be reduced if the viscosity of engine oil becomes too low. Similarly, when the viscosity of the oil is higher, friction between lubricated parts is higher, but engine durability may be increased. Thus, an optimum engine oil viscosity may be determined based upon engine operating conditions that offers a proper balance of limiting friction between lubricated parts, and offering acceptable engine durability. Therefore, a need exists for an engine oil system that monitors engine oil viscosity, and controls the temperature of the engine oil to adjust the engine oil viscosity to a predetermined level based upon engine operating conditions.